1. Technical Field
The present invention relates to an optical transmission system and an optical transmission method.
2. Background Art
In recent years, optical transmission and reception systems for transmitting and receiving transmission code sequences, or the like, at high speed by using optical transmission lines are being developed.
The orthogonal polarization multiplexing and digital coherent reception method have received attention as technology to utilize the optical transmission and reception system.
The orthogonal polarization multiplexing is a method utilizing the property of light as an electromagnetic wave and multiplexing a signal channel by utilizing polarization. To be more specific, since light includes a transverse wave which includes two polarized components that are orthogonal with respect to the direction of the light, the orthogonal polarization multiplexing multiplexes the two polarized components by adding independent information thereto in order to transmit a larger amount of information.
The digital coherent reception method is a method applying a digital signal processing on a coherent optical communication technology. For example, a reception side receives information by using a beat signal generated from the interference between a transmitted light signal and a local oscillator.
In Japanese Unexamined Patent Application Publication No. 2011-188044, an optical transmission system, in which a signal is transmitted upon separating an X polarized signal and a Y polarized signal from one another, is disclosed. Further, according to such system, of the two polarized signals, a determination is made as to which is the X polarized signal or the Y polarized signal.
In Japanese Unexamined Patent Application Publication No. 2009-027525, an optical transmission system is disclosed in which, at a transmission device a light signal is phase modulated and directions of adjacent bits are orthogonalized, whereas at a reception device, a light signal and another light signal which is delayed by an even number worth are made to interfere with one another in order to perform a delay detection.
In Japanese Unexamined Patent Application Publication No. 2010-050735, a differential phase shift keying optical transmission system is disclosed. According to such system, by synthesizing slots of two adjacent pulses having the same polarization state even when light pulses partially overlap with one another no bit error would occur.
In Japanese Unexamined Patent Application Publication No. 61-280136, a digital communication device which improves the code error rate using a DSK method is disclosed.